(i) Field of the Invention
This invention relates to colour-coded insulating materials which can be dielectrically matched and their manufacture; more especially the invention is concerned with a process for preparing a particulate concentrate for the manufacture of insulating materials; with a set of separately packaged colour-coded particulate concentrates and with methods for producing a colour-coded, cellularly insulated cable conductor wire and a multi-conductor cable, more especially a communication cable.
(ii) Description of the Prior Art
Insulated cables, for example power and communication cables generally comprise a plurality of insulated conductors enclosed within a waterproof sheath; such cables may consist of groups of two or more such insulated conductors twisted together, for example, a twisted telephone pair or a quad. A communication cable may consist of a plurality of such twisted pairs or quads twisted together.
Frequently the conductors are covered or coated with a cellular insulation and such insulated conductors may be produced by extruding around a conductor wire a thermoplastic polymer containing gas bubbles, produced by the thermal decomposition of a solid chemical blowing or foaming agent or by direct injection of a gas into the polymer in the presence of a nucleating agent and allowing the gas bubbles to expand thus producing the cellular structure.
It is frequently necessary to employ readily indentifiable conductor wires within a cable, and this ready identification may be achieved by the employment of different colours for different wires, so that the wires are colour-coded. By the employment of different colour combinations individual singles, pairs or quads within a group can be readily identified, for example, one pair might comprise a red wire and a blue wire, a second might comprise a red wire and a white wire, a third might comprise a red wire and a green wire and so on.
The different colours are obtained by the employment of appropriate amounts of pigment. It is found, however, that different pigments have different effects on the electrical characteristics of the extruded insulating coatings and in particular different pigments affect the dielectric constant of an insulating polymer to different degrees. Further, the impairment in the dielectric constant is not necessarily linear with respect to the degree of pigmentation.
Differently pigmented polymers are not suitable for insulation of conductors in communication cables if they are electrically mismatched, since, for example, in a telephone cable any mismatching results in crosstalk which is difficult to eliminate.
Systems of electrically matched colour-coded compositions have been developed to overcome this problem of mismatching, which compositions employ particular concentrations of particular pigments. One such system which has found wide acceptance is that described in Canadian Pat. No. 616,767 of John B. Howard and Vincent L. Lanza, issued Mar. 21, 1961, the disclosure of which is herein incorporated by reference. Canadian Pat. No. 616,767 describes eleven colour-coded compositions for producing multi-conductor communication cables in which different coloured insulating coatings are dielectrically matched. One of the colour compositions, namely the gold, has since become obsolete and there are now conventionally employed ten colour-coded compositions based on the compositions described in the aforementioned Canadian Patent.
Thus there are conventionally employed ten visually distinctive colour compositions namely blue, orange, green, brown, slate, white, red, black, yellow and purple.
The present invention is concerned with improvements in the employment of dielectrically matched colour-coded compositions such as those described in the aforementioned Canadian Patent which permits the production of colour-coded, dielectrically matched cellular insulations.
The aforementioned Canadian Patent was concerned primarily with solid insulation coatings, however, with the increasing popularity of cellular insulating coatings in communication cables the same colour-coded systems have been employed in the production of cellular insulations.
In conventional practice the blowing agent and the polymer for the insulation coating are premixed in predetermined amounts having regard to the desired cellular structure and formed into pellets which are added to the screw extruder separately from the pigment.